1. Field of the Disclosure
The disclosure is related to the field of electromagnetic induction well logging for determining the resistivity of earth formations penetrated by a wellbore. More specifically, the disclosure relates to measuring the transient signals in an induction tool in the presence of vibration of the logging tool used for making measurements.
2. Description of the Related Art
Electromagnetic induction resistivity instruments can be used to determine the electrical conductivity of earth formations surrounding a wellbore. An electromagnetic induction well logging instrument is described, for example, in U.S. Pat. No. 5,452,761 issued to Beard et al. The instrument described in the Beard '761 patent includes a transmitter coil and a plurality of receiver coils positioned at axially spaced apart locations along the instrument housing. An alternating current is passed through the transmitter coil. Voltages which are induced in the receiver coils as a result of alternating magnetic fields induced in the earth formations are then measured. The magnitude of certain phase components of the induced receiver voltages are related to the conductivity of the media surrounding the instrument.
The development of deep-looking electromagnetic tools has a long history. Such tools are used to achieve a variety of different objectives. Deep looking tools attempt to measure the reservoir properties between wells at distances ranging from tens to hundreds of meters (ultra-deep scale). There are single-well and cross-well approaches, most of which are rooted in the technologies of radar/seismic wave propagation physics. This group of tools is naturally limited by, among other things, their applicability to only high resistivity formations and the power available downhole.
Deep transient logging while drilling (LWD), especially “look-ahead” capability, was shown to have a great potential in predicting over-pressured zones, detecting faults in front of the drill bit in horizontal wells, profiling massive salt structures, etc. One of the main problems of deep transient measurements in LWD application is a parasitic signal due to the conductive drill pipe. A variety of techniques have been used to reduce this parasitic signal in the acquired data. For the purposes of the present disclosure, we adopt the following definition of the term “Transient Electromagnetic Method” from the Schlumberger Oilfield Glossary:                A variation of the electromagnetic method in which electric and magnetic fields are induced by transient pulses of electric current in coils or antennas instead of by continuous (sinusoidal) current.        
Many of the applications for TEM involve depths of investigation of up to 100 m. A solution for the high energy excitation source for deep transient measurements has been disclosed in US 20050189945 of Reiderman. However, high sensitivity induction receivers are known to suffer from impulsive noise due to motion of the inductive coil in the earth's magnetic field. Methods have been proposed for cancellation of acoustic noise in acoustic LWD tools. See, for example, U.S. Pat. No. 6,470,245 to Dubinsky where the cancellation signal is derived from the time-reversed tool mode and formation mode signals.
The present disclosure is directed towards methods for removal of low frequency (below about 1 kHz) noise in TEM signals caused by drill string vibration in the earth's magnetic field.